This invention relates to a cordless telephone system for use in a building with one or more moving conveyances, such as an office building with elevators or a factory with moving overhead cranes.
Cordless in-building telephone systems have been available for several years. A typical system comprises a central switch coupled by wires to external lines, to extension telephones installed in rooms of the building, and to fixed base stations installed in rooms and corridors of the building. Hand-held portable telephones communicate via wireless links with the base stations, thus enabling calls to be set up between a pair of portable telephones, or a portable telephone and an extension telephone, or a portable telephone and an outside telephone.
In existing systems a problem occurs when a person making a call with a portable telephone enters a moving conveyance such as an elevator. Initially, the call is relayed by a base station near the entrance to the elevator, but the elevator rapidly carries the person away from this base station, thereby weakening the wireless link between the base station and the portable telephone. As the separation increases the quality of the link deteriorates, making communication difficult, and when a certain separation is reached the link is broken, cutting off the call. Reliable telephone communication from a moving elevator is thus impossible.